Purported Synthesis of 3,4,7,8‐Tetraphenyl‐1,2,5,6‐tetraazocine from Benzil and Hydrazine: Competing Cyclization and Carbon–Carbon σ‐Bond Scission

Abstract

AbstractThe claims that 3,4,7,8‐tetraphenyl‐1,2,5,6‐tetraazocine can be prepared by the thermal condensation of two moles of benzil monohydrazone or of an equimolar mixture of benzil and benzil dihydrazone have been thoroughly reinvestigated. When such thermolyses were conducted in moist air, neither the claimed 3,4,7,8‐tetraphenyl‐1,2,5,6‐tetraazocine nor the precedented isomeric tetraazapentalene derivative was detected. The following products were unambiguously formed from the heating of molten benzil monohydrazone (%): benzil (10), benzaldehyde (10), benzamide (22), benzyl phenyl ketone (19), benzil bis(ketazine) (11), 3,4,5,6‐tetraphenylpyridazine (9), benzil benzaldehyde azine (10), and, after column chromatography, 2,4,5‐triphenylimidazole (2). This last component had a melting point and the fluorescent properties in UV light attributed by the original investigator to the mistakenly presumed 3,4,7,8‐tetraphenyl‐1,2,5,6‐tetraazocine. Thus, the original claims for the synthesis of such a novel tetraazocine ring or even for the synthesis of the precedented isomeric zwitterionic tetraazapentalene have now been repudiated. The formation of 2,4,5‐triphenylimidazole as a side reaction in the thermolysis of benzil monohydrazone can readily be rationalized as arising from benzil, benzaldehyde, and a source of ammonia, namely, benzamide, in the long‐known Radziszewski reaction. Corroborating evidence was provided by data from the thermolysis of benzil dihydrazone. In addition, the origin of other side products is explained in terms of other possible condensations. Finally, preliminary experiments on using irreversible dehydrating agents such as titanium(IV) isopropoxide with benzil monohydrazone indicate that 3,4,7,8‐tetraphenyl‐1,2,5,6‐tetraazocine is formed at room temperature as a transitory intermediate, which eliminates dinitrogen to produce 3,4,5,6‐tetraphenylpyridazine.(© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008)